In recent years a hierarchical virtual storage system in which an inexpensive large-capacity record medium, such as a magnetic tape, is used as a back-end library apparatus and in which a record medium, such as an HDD (Hard Disk Drive), access to which can be gained at a higher speed is used as a cache apparatus has become known. With a virtual storage system data stored in a cache apparatus looks like data in a library apparatus from a host apparatus. As a result, the host apparatus can use a large-capacity storage area provided by the library apparatus as if it were connected to the host apparatus.
Furthermore, with some virtual storage systems a logical volume generated on a magnetic tape contained in a library apparatus can be recorded on an external save magnetic tape and be saved outside. The function of ejecting a magnetic tape on which a logical volume is recorded to the outside is referred to as, for example, an “export function”. With some virtual storage systems having the export function, for example, mounting a record medium on a library apparatus and ejecting a record medium from the library apparatus are managed by the set including a plurality of record mediums, and a logical volume is managed by associating it with a set.
Another example of a storage apparatus is as follows. An ejection opening and an insertion opening for a virtual tape are virtually prepared and the virtual tape is moved to the ejection opening. When instructions to output the virtual tape to a removable medium are given, the virtual tape in the ejection opening is written to the removable medium and the virtual tape is moved to the insertion opening.    Japanese Laid-open Patent Publication No. 2006-172400    Japanese Laid-open Patent Publication No. 2007-293778
By the way, in order to realize the above export function, a logical volume which is an export object is copied from a cache apparatus to an external save magnetic tape. If the logical volume which is an export object is not stored in the cache apparatus, the logical volume is read out from a magnetic tape to the cache apparatus and the logical volume read out is written to the external save magnetic tape.
If reading out data from the magnetic tape to the cache apparatus is necessary, there is a possibility that while the data is being read out, the operation of writing to the external save magnetic tape will be discontinued halfway. A state in which the operation of writing to the external save magnetic tape is discontinued halfway means a state in which the magnetic tape that is not being read or written occupies a tape drive. Such a state contributes to a fall in the efficiency of the use of the tape drive. As a result, a long time may be taken to perform an entire export process.